Thanks for the response. Can you suggest a course of action? I presume I'll need to take off anything loose, clean up and apply a new render but what would that render be? Another mix of concrete and sand?
I'm in Scotland and I have some mullions and sills falling apart and I'm not sure what service I even need. You can see from the second image the entire top of the sill would come away if I knocked it hard enough so there's quite a bit needs to be taken out and rendered. Someone suggested just...